RESISTANCE BY LEAVES TO WATER LOSS 



13 



the laboratory for a few minutes at each weighing. The differ- 

 ent operations were always carried out in the same order, and 

 there occurs in this series but Uttle discrepancy between the 

 actual time at which any observation was taken and that which 

 would have been requisite to give an hourly rate. The errors 

 produced by slight variations in the length of the time periods 

 have been ignored in this experiment. 



Table 1 presents samples of the original data from the paper 

 tests, for hours 21 and 7, and also shows the manner of manipu- 

 lating these data in the derivation of the indices of transpiring 

 power for the two leaf surfaces and for the leaf as a whole. 



TABLE 1 





21 





Time required for paper response, seconds : 







Over water 





13 



18 





[Test 1 



600 



480 



Upper leaf surface 



Test 2 



840 



420 





Average 



720 



450 





[Test 1 



2100 



300 



Lower leaf surface 



Test 2 



3000 



300 





Average 



2550 



300 



Index of transpiring power, ratio of water test 







time to average leaf time: 







Upper leaf surface 







0.018 



0.040 



Lower leaf surface 





0.005 



0.060 



Average index for whole leaf 



0.012 



0.050 



In table 2 (columns 2, 5 and 8) are given the indices of tran- 

 spiring power for the two leaf surfaces, and the average indices 

 for the entire leaf surface, for each observation. To obtain the 

 average indices of transpiring power for each hour period, the 

 indices from each two consecutive observations have been aver- 

 aged, and the averages thus derived are also included in this 

 table (columns 3, 6 and 9). Furthermore, the values of the last- 

 named indices after reduction to a uniform basis for comparison 

 (the index for hour 1), are Ukewise included (columns 4, 7 and 10). 



It is e\'ident at once from table 2 that the index of transpiring 

 power varies definitely with the different hours of the day, and 



